During oilfield operations, tubing or wireline or electric line can be used to place a wide variety of downhole tools in a wellbore. A disconnect mechanism is necessary should the equipment being run into the well become stuck. While a tubing string can withstand substantially higher extraction forces than wireline or electric line, many times operators prefer to run wireline because it saves substantial rig time in getting the downhole tools positioned properly in the wellbore. In the past, disconnect mechanisms have been provided which primarily rely on shear pins. Since wireline or electric line has fairly low tensile capabilities with respect to a tubing string, the shear screw or screws used in the prior art had to be set at a fairly low shear rating. The low shear rating was necessary to prevent damage to the wireline or electric line from excessive tensile stress should the downhole tool become stuck in the wellbore.
However, problems have been encountered using a shear screw or screws that have a low failure point. During normal operation, the shear screws are exposed to various cyclical forces which tend to affect their ultimate shear rating. The shear screws are also exposed to the fluids in the wellbore which also over time can affect the inherent strength of the shear screws or pins, making them susceptible to failure at stresses below their rated failure point. Unexpected release can significantly delay operations, thereby costing the well operator significant sums due to the delays incurred. Unexpected release of a release mechanism can also result in loss of the downhole tool in the wellbore and in extreme cases can cause severe damage to the wellbore, which requires substantial time and money to repair.
It has long been desired in wireline or slickline applications to have a release mechanism that will predictably release with a known preset force. Such a release mechanism would ideally be able to provide numerous cycles of operation, with reliability of performance so that premature release would not occur. One of the objects of the present invention is to provide a simple, easy-to-construct release mechanism which will operate reliably at a desired release force. Another object of the present invention is to provide a simply constructed release mechanism which is so configured as to be substantially unaffected by the wellbore conditions or prior cycles of loading. Another object of the invention is to provide a simply constructed release mechanism which can be easily reused without significant disassembly and reassembly.